EMBEDDING 71 



hardening gelatine gels. It is made by diluting commercial forma- 

 lin with 4 times its volume of a 5 °o aqueous solution of potassium 

 alum crystals. Gelatine blocks may be preserved indefinitely in 

 formalum. 



Formaldehyde solution should be used without the addition of 

 alum if for any reason it is necessary to avoid the mordanting 

 eifect of aluminium salts (for instance, in the acid haematein 

 test^^ for phospholipids ; on the subject of mordanting, see p. 1 10). 



Beyond hydrogen bonding and the formation of covalent links, 

 a third hardening process is generally used before sections are cut. 

 The water contained in the fixed gelatine is frozen, usually by 

 allowing carbon dioxide at a low temperature to flow past the 

 block. If, however, the gelatine gel has been hardened by evapora- 

 tion, little water remains in it and the block can be cut with a glass 

 knife without cooling below room-temperature.®^ 



It is possible to attach gelatine sections to glass slides,^" but it 

 is simpler to dye them (or to treat them with lysochromes) while 

 they are still loose. If loose sections are mounted in an aqueous 

 mounting medium, such as dilute glycerine (p. 128) or Farrants's 

 medium (p. 129), the tissue will have been kept continuously wet 

 with water from the moment when the cells were still alive. 



PARAFFIN 



Solid paraffin (so-called paraffin 'wax') is more commonly used 

 than any other embedding medium. It certainly has great 

 advantages. The process of embedding is quick and simple; em- 

 bedded material may be stored indefinitely in the dry condition; 

 sections may be obtained regularly at all necessary thicknesses 

 from about 2 fx upwards, if suitable waxes are chosen; and each 

 section adheres automatically to the next as it comes off" the 

 microtome knife, so that ribbons are formed. These qualities 

 make paraffin the best embedding medium for most purposes in 

 micro-anatomy and routine histology, and it plays an important 

 part also in studies of chromosomes and other cellular con- 

 stituents that are not easily distorted or dissolved. The main 



